1. Field of the Invention
The present invention relates to a paper feeding device which is employed as a paper feeding mechanism in a printing apparatus such as an inkjet printer, and more particularly, to the paper feeding device which is so designed as to stock paper by mounting a plurality of sheets of paper in a stack on a paper mount erected in a slanted shape.
2. Description of the Related Art
FIG. 2 is a schematic perspective view for explaining a structure of a paper feeding device of this type. In the drawing, reference numeral 1 designates a base part, 2 designates a paper mount which is divided into a stationary slanted part 21 formed of synthetic resin integrally with the base part 1, and a movable slanted part 22 foldably connected to this stationary slanted part 21. Moreover, a paper feeding load applying unit 3 which is called as “a buckler friction” is provided at one position in a lateral direction of the base part 1, and a paper feeding roller 4 composed of a pickup roller is provided above this paper feeding load applying unit 3. Paper edges 101 of a plurality of sheets of paper 100 which are mounted in a stack on the paper mount 2 are supported by the base part 1 and the paper feeding load applying unit 3.
By the way, in the paper feeding device of this type, when the paper feeding roller 4 has come into contact with an uppermost sheet of paper (a first sheet) among the plurality of sheets of paper 100 mounted in a stack on the paper mount 2 thereby to rotate, a feeding force by the paper feeding roller 4 will be directly applied to the first sheet. On the other hand, to a second sheet lying under the first sheet and a third sheet lying under the second sheet, feeding forces which are caused by friction with the respective overlying sheets will be given. In view of the above, in the paper feeding device of this type, loads for restraining the second and third sheets of paper from being fed with the feeding forces caused by the friction between the sheets are given to the second, third and succeeding sheets, for the purpose of preventing such multiple feed that the second and third sheets of paper may be also fed, by the friction, following the first sheet. The above described a paper feeding load applying unit 3 serves to provide a function of generating such loads (paper feeding loads).
Meanwhile, as illustratively shown in FIG. 3, the paper mount 2 is erected in a slanted shape with respect to the base part 1. Therefore, when the sheets of paper 100 are mounted in a stack on the paper mount 2 having their paper edges 101 supported by the base part 1 and the paper feeding load applying unit 3, erection angles of the sheets of paper 100 with respect to a surface of the paper feeding load applying unit 3 is generally larger in the lower sheets which are close to the paper mount 2 than in the upper sheets which are remote from the paper mount 2. For example, provided that the erection angle of the first sheet of paper 100 is θ1, and the erection angle of the lowermost sheet of paper 100 is θ2, the angle θ1 is smaller than the angle θ2, in many cases. When the erection angles of the two sheets overlapped on each other are compared, the erection angle of the upper sheet is equal to or smaller than the erection angle of the lower sheet, in many cases.
On the other hand, FIG. 4 shows the paper feeding load applying unit 3 employed in the conventional paper feeding device. As shown in the drawing, this paper feeding load applying unit 3 has an elongated surface of rubber material on which convex portions 31 in a serrated shape having a same height are formed along its entire length, at an equal interval in a longitudinal direction. The paper feeding load applying unit 3 having such structure is provided in a case 32 which is mounted on the base part 1 described referring to FIG. 2, and the convex portions 31 of the paper feeding load applying unit 3 serve to generate the above described paper feeding load for preventing the multiple feed. More specifically, it is so constructed that although the paper edge 101 of the first sheet of paper 100 to which the feeding force has been directly given by the rotation of the paper feeding roller 4, as shown in FIGS. 2 and 3, can overcome the paper feeding load given by the convex portions 31 of the paper feeding load applying unit 3 to go across the convex portions 31, the paper edges 101 of the second and third sheets which have received the feeding force by the friction between the sheets will be influenced by the paper feeding load given by the convex portions 31 of the paper feeding load applying unit 3, and will be unable to easily go across the convex portions 31.
There has been known another conventional example, in which a part of an upper face of the case 32 described referring to FIG. 4 is made higher, so that the heights of the convex portions 31 of the paper feeding load applying unit 3 with respect to the upper face of the case 32 may be relatively smaller in an area close to the paper mount than in an area remote from the paper mount (See Japanese Utility Model Registration No. 3092259, for example). There has been known still another conventional example, in which for the purpose of preventing the multiple feed of the sheets of paper which have been mounted on a paper feeding cassette of lateral mounting type in an image forming apparatus, a slope separating plate is provided at a paper discharging side of the paper feeding cassette, and means for applying a load to the discharged sheet thereby to prevent the multiple feed is provided at an upper end part of the slope separating plate (See JP-A-2003–128288, for example).
It has been found that, in the conventional paper feeding device provided with the paper feeding load applying unit 3 as described referring to FIG. 4, magnitudes of the feeding loads given to the sheets of paper by the convex portions 31 of the paper feeding load applying unit 3 depend on the erection angles of the sheets of paper 100 with respect to the surface of the paper feeding load applying unit 3, and that a larger paper feeding load will be generated in case where the erection angle is larger than in case where the erection angle is smaller. For this reason, in the conventional paper feeding device provided with the paper feeding load applying unit 3 as described referring to FIG. 4, even though the upper sheets of paper 100 having the smaller erection angles are normally fed one by one by the rotation of the paper feeding roller 4 without causing the multiple feed, normal paper feeding motion will not be performed, in some cases, with the lower sheets of paper 100 having the larger erection angles, because the paper feeding load given by the paper feeding load applying unit 3 has become too large. According to cases, the paper edge 101 of the sheet of paper 100 to which the feeding force by the paper feeding roller 4 has been given may be butted against the surface of the paper feeding load applying unit 3 to be folded in a crushed state, and so-called paper jam may happen.
In this respect, according to the paper feeding load applying unit which is disclosed in Japanese Utility Model Registration No. 3092259, the paper feeding motions are conducted without occurrence of the paper jam in all the sheets of paper stacked on the paper mount, because the paper feeding load to the upper sheets having the smaller erection angles becomes larger, while the paper feeding load to the lower sheets having the larger erection angles is depressed to be smaller. However, in this case, the upper face of the case to which the paper feeding load applying unit is mounted must be modified in shape to attain the purpose, and there has been a problem that a structure of a mold for molding the case has become complicated, and a rise of production cost has been inevitable.
On the other hand, the paper feeding load applying unit which is disclosed in JP-A-2003-128288 has merely such a structure that the area for generating the load is formed at the upper end part of the slope separating plate, for the purpose of preventing the multiple feed of the sheets of paper mounted on the paper feeding cassette of the lateral mounting type. Therefore, it is not sufficiently effective as means for preventing the multiple feed, under circumstances where every sheet of paper has a different erection angle, as described referring to FIG. 3.